1. Field of the Invention
The present invention relates to polymers based on polyalkyl (meth)acrylates, for use as bitumen improving agents; and to bitumen modified with such polymers and having improved engineering properties.
2. Description of the Background
Bitumen, which is a mixture of essentially different hydrocarbons and which contains varying amounts of paraffinic, naphthenic, and aromatic hydrocarbons, has properties which make it useful in a number of applications including its use as a component in road surfaces, its use as a sealing compound, its use as a coating material, its use in the preparation of tar paper and the like. It can also be used to protect building structures, and as a caulking or waterproofing material or the like, to protect against ground water.
Bitumen generally has little or no elasticity. Because of its inherent properties, coatings or pavement layers comprised of bitumen are brittle at low temperatures and soft at higher temperatures.
Attempts have been made to use various additives, particularly polymers, to improve bitumen, particularly to improve its rheological properties such that, e.g., if an asphalt is prepared from a mixture of polymer, bitumen and minerals and then used as a roadbuilding material, the resulting prepared roadbed has sufficient stability and cold flexibility, as well as high fatigue strength under constantly increasing traffic loads. Polymer addition can increase the so-called plasticity range, i.e., the difference between the softening point and the failure point. If the failure point (DIN 52 012) is lowered and the ductility (DIN 52 013) is increased, improvement of the cold flexibility is indicated. The objective of using polymers as bitumen additives is to alter the viscoelastic properties of bitumen, in particular to extend the plasticity range and to improve the elastic restoration.
In addition to natural and synthetic rubbers, other polymers which have been tested as bitumen improvers are duroplastic resins and thermoplastic resins (Koerner et al, 1977 Plaste und Kautschuk, 24, 475-478). Principal polymers currently used as bitumen additives are styrene/butadiene copolymers (SB), EPDM copolymers, ethylene/vinyl acetate copolymers (EVA), and atactic polypropylene, the latter being used particularly for industrial bitumen. Polymer-modified bitumens available commercially include Carabit .RTM. (SB, supplied by Shell), Olexobit .RTM. (EPDM, supplied by BP), and Styrelf.RTM. (SB, supplied by Elf).
Other known bitumen improvers are polymers based on (meth)acrylic acid esters, particularly polymers of alkylmethacrylic acid esters (PAMAs). These polymers are still limited in their applicability, despite the fact that they satisfy a number of criteria as additives. The principal drawback which they have is that they do not sufficiently improve the elastic restoration of the finished elastomer-modified bitumen.
U.S. Pat. No. 2,420,082 describes bitumeniferous mixtures comprised of bitumen and 2-5% of polyalkyl methacrylates wherein the alkyl group has 8-10 C atoms. The ductility of the bitumen is improved. Specific data on the molecular weights are not given.
Also, U.S. Pat. No. 2,848,429 describes bitumeniferous mixtures with improved ductility, obtained by addition of rubber and polyalkyl (meth)acrylates, each in the amount of 0.01-0.5%. The PAMAs have molecular weights of 5,000-500,000.
Ger. AS 12 40 773 and OS 23 31 727 also describe improved bitumeniferous binders for road surfacing, which contain alkyl methacrylate polymers. No detailed or specific information about the polymers is given.
Polyalkyl (meth)acrylates and copolymers of alkyl (meth)acrylates have also been proposed as improvers for bitumen. The polymer described in Ger. OS 25 09 473 is a solid mixed polymer with molecular weights of 50,000-500,000. In Jap. OS 52-141,829, stagewise emulsion polymerized additives are described, and in PCT 88/07 067, polyalkyl methacrylates wherein the alkyl groups have 4-20 C atoms and having molecular weights of c. 5,000-1,000,000 are described which are contained in amounts of 0.01 to c. 10 wt. % in the modified bitumen.
Criteria which polymer candidates as additives should possess if they are to improve the properties of bitumeniferous mixtures are oxidation resistance, thermal stability, ease of incorporation into bitumen, compatibility with different types of bitumen, and stability of the resultant bitumen-polymer mixture.
Known polymeric bitumen additives adequately improve cold flexibility in bitumeniferous mixtures and resulting mineral mixtures. Thus, the failure point and ductility of such materials are favorably affected by addition of known additives. However, the fatigue strength and elasticity of asphalts obtained with the known bitumeniferous binders are inadequate. These properties are determined by the elastic properties of the bitumen. An indication of failure is the appearance of deformed asphalt road surfaces. A need therefore continues to exist for an improved bitumeniferous binder.